1. Field of the invention
The present invention relates to a method for manufacturing a superconductor device, and more specifically to a method for forming an oxide superconductor thin film having different thickness portions, in a process for manufacturing a superconductor device.
2. Description of related art
In a process of manufacturing superconductor devices such as Josephson devices and superconductor transistors, it is necessary to form a superconductor in the form of a thin film, and to work the superconductor thin film into a desired shape.
When a superconductor used in the superconductor devices is composed of an oxide superconductor such as Y--Ba--Cu--O oxide superconductor typified by Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x, Bi--Sr--Ca--Cu--O oxide superconductor typified by Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y, and Tl--Ba--Ca--Cu--O oxide superconductor typified by Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z, formation and working of the thin film have been performed as follows:
In an ordinary practice, a thin film of an oxide superconductor is formed by using a physical vapor deposition process such as a sputtering and molecular beam epitaxy, or a chemical vapor deposition process such as metal organic chemical vapor deposition (MO-CVD). After formation of the thin film, a heat treatment can be performed in order to improve characteristics of the oxide superconductor thin film. Regardless of the kind of the thin film formation process, it is necessary to unify the crystal orientation in the formed oxide superconductor thin film. The reason for this is that the above mentioned oxide superconductor generally has directivity in superconduction critical current density.
In the case of forming an oxide superconductor thin film having different thickness portions, a thin film having a thickness equal to the thickness of the thinnest thickness portion of the different thickness portions is first formed, and thereafter, a mask is deposited on a portion to be left as the thinnest portion, and an oxide superconductor is further deposited on the thin film having a thickness equal to the thinnest thickness.
In the following, there will be explained an example of a conventional process for forming on a substrate an oxide superconductor thin film having a central portion of 400 nm thickness and opposite side portions of 100 nm thickness. First, an oxide superconductor thin film having a thickness of 100 nm is formed on the substrate by sputtering or another process. A portion of the thus deposited oxide superconductor thin film other than a central portion is masked with a photoresist, and an oxide superconductor is further deposited by sputtering or another thickness, so that the central portion is constituted of an oxide superconductor thin film having a thickness of 400 nm.
As seen from the above, the conventional process is featured in which an oxide superconductor thin film having a thickness of the thinnest thickness portion is formed on a whole surface of the substrate, and thereafter, an unnecessary portion of the formed oxide superconductor thin film is covered with a mask and an oxide superconductor is selectively further deposited on only a required portion of the formed oxide superconductor thin film. As a result, the oxide superconductor thin film having different thickness portions is formed.
The above mentioned process has been disadvantageous in that reaction occurs at an boundary between a photoresist used as the mask and a firstly formed oxide superconductor thin film, so that the characteristics of the oxide superconductor is deteriorated. In a photolithography process, a surface of the first formed oxide superconductor thin film is in contact with an alkaline developing liquid, a resist removing liquid and a cleaning water. An upper oxide superconductor layer is deposited on the surface of the lower oxide superconductor thin film which had once been in contact with these kinds of liquid. In this case, a discontinuous boundary is formed between the upper oxide superconductor layer and the lower oxide superconductor thin film, and a uniform oxide superconductor thin film having a thick portion and a thin portion cannot be obtained.